\section{Introductory Knowledge}

\subsection{An example}
The University of Twente uses RFID readers and tags to restrict access to certain locations. All employees and students at the University of Twente have an employee or student card. This card has a passive tag build into it which is capable of communicating with a reader.

Take the unlocking of a door using a smart card for example. In this case the reader is a device at the door that will unlock the door when an authorized tag is presented in front of the device. The reason why a tag is called passive is because it does not contain a power supply and will only become activated in the proximity of a reader, normally this is up to 10 cm \cite{wiki:MIFARE}. Primarily this distance is kept so short to prevent people from unlocking doors by simply walking past them. However this short distance also gives the false feeling that nobody is able to interfere with the signals being send between reader and tag.

The reader will detect a tag in it's proximity. When it does, it first asks the tag for its type. It does this to make sure the rest of the transmission goes according to the protocol as outlined in the ISO/IEC 14443 specification. The tag will respond by giving its type, after which the reader directly asks for the tags identification number. The identification the tag gives is checked against a database to see if this tag is authorized to unlock the door. The reader could then ask for more information that may be stored on the card. But for unlocking the doors at the University of Twente only the identification number is needed.

\subsection{The link}
The reader and tag communicate using the concept of Near Field Communcation (NFC)\cite{RS:MFCTM}. The reader contains a coil which is used to create a magnetic field. The tag also has a coil which can capture these fields. Together both coils form a weak coupled inductor pair. A well known concept of a coupled inductor pair is that the load on the secondary side can be seen on the primary side. This the principle of operation in NFC. The reader sends out a carrier wave of $13.56 MHz$\cite{RS:MFCTM}. This carrier wave is the basis for the communication link but also provides the power for the tag to operate. 

Two modulation techniques are used to establish the communication link of NFC.
The reader communicates to the tag using On-Off Keying (OOK) modulation \cite{RS:MFCTM} where the carrier is suppressed completely to represent a binary zero this is shown in \cref{fig:OOKModulation}. During these off times the passive tag should have stored enough energy to keep operating.

\begin{figure}[ht!]
\centering
\includegraphics[width=10cm]{assets/forwardchannel.png}
\caption{Forward channel OOK modulation}
\label{fig:OOKModulation}
\end{figure}

The tag can communicate back to the reader by modulating the load on its coil (\cref{fig:LoadModulation}). It does this by using Amplitude Shift Keying (ASK) modulation \cite{RS:MFCTM}. The varying load of the tag is visible at the reader side as a varying amplitude as shown in \cref{fig:ASKModulation}.

\begin{figure}[ht!]
\centering
\includegraphics[width=10cm]{assets/backwardchannel.png}
\caption{Backward channel ASK modulation}
\label{fig:ASKModulation}
\end{figure}
